Olympus and Panasonic are moving the micro four-thirds market along very nicely with their Pen E-P1/2 and GF1 offerings but relying on the main sensor to do (contrast based) autofocus seems to be a compromise. The rationale behind getting rid of the mirror assembly and pentaprism was to save space but I'm left wondering why they didn't take the option of using a much smaller partially "silvered" mirror to divert some light to a conventional phase change AF sensor. ¹

Or maybe, with sufficiently fast processing, contrast based AF can match phase change? Not having had a chance to try out the GF1, which is apparently about as good as contrast based AF gets at the moment, I have no clear idea of what the current shortfall is. Would any GF1 owners like to comment?

Bob.

¹ No need for full-size mirrors and pentaprisms and it seems likely that a small lightweight mirror positioned fairly close the the rear of any lens could not only be withdrawn quickly (sideways?) just prior to capturing each image but could, with careful design and some electronic trickery, be made virtually invisible in the live view screen or electronic viewfinder. Not simple, I suppose, as one would have to use variable gain to compensate for the shading effect and one would need a way to calibrate third-party lenses (shooting a white card, perhaps).

While the AF of the GF1 is claimed to be fast (I also have not used it), I have to wonder if that is only for "single shot" use? Is it as good as phase for tracking a moving subject? I have used a Panasonic ultra-compact, and the AF on that is faster than on the E-P1.

The idea of using a fixed mirror for phase AF recently turned up in a Sony rumour. The proposal was that you had a fixed semi-silvered mirror redirecting part of the light to AF sensors, with the rest going to the main sensor for use with EVF.

Question: how much light is redirected to the AF detectors in a DSLR? Obviously not all of it, otherwise the OVF will be rather useless. Let's assume half the light for simplicity. Assuming a fixed mirror (for mechanical simplicity), would you trade one stop loss for faster AF in a compact sized camera?

There is one further cost of course. With a mirror in place, you need to increase the space between sensor and lens, which would compromise the body size.

Bob, if I read your proposed method correctly, I would caution a little in that I'm not sure how much spatial information is required for phase to work, as that seems to be more important than absolute level.

Thanks for the feedback. I think that having a fixed mirror might be too costly in terms of light loss during image capture as well as in wasted space. But my own proposal of a small removable mirror would have drawbacks as well - maybe that's why nobody has done it.

But I would really love this class of camera to be able to compete in every way with full sized DSLRs. With a fully integrated high definition electronic viewfinder, with the eyepiece positioned at the top left of the back of the camera, and an ability to use that viewfinder to also review what one has just captured one could even dispense with the use of that rear LCD, though I'm sure the marketing crowd would insist it be retained.

But I'm still curious to know from GF1 owners who also have a DSLR how the GF1 AF fares in comparison.

Not having had a chance to try out the GF1, which is apparently about as good as contrast based AF gets at the moment, I have no clear idea of what the current shortfall is.

It's my understanding that the title of fastest contrast based AF in an interchangeable lens camera is currently held by the GF1's predecessor, the GH1, which averages around 1/3 of a second (while the GF1 is around 0.1 sec. slower).

As to the other question of phase change vs light loss, given the current lack of fast (affordable) (prime) lenses for the m4/3 format, I personally would prefer to stick with contrast based AF. Especially if phase change would also add to the body size since that's one of the format's distinguishing characteristics.